Prior art FCC sample systems often plug with catalyst fines from the flue-gas. The frequency of complete plugging depends upon the type of sample system used. One refinery's sample system worked for two to four weeks at a time, followed by four to eight hours of maintenance. Another refinery's sample system worked for only eight to ten hours at a time.
When the sample system plugs, oxygen measurements read high and carbon monoxide and carbon dioxide measurements read low. This results from air entering the analyzer and diluting the flue gas sample. Under normal conditions, ejectors draw sample through the analyzer, but when the sample system fails, they draw air. While this source of failure can be detected with a low-flow switch on the sample line, the measurement readings will still be inaccurate.
Prior sample systems for gas streams with particulate matter are not reliable when applied to FCC flue-gas sampling. FCC catalyst fines are noted for setting-up like cement when wetted and dried. One very simple sample system uses a small diameter pipe to receive and cool the sample, but this pipe plugs after eight to ten hours of use. Another more complicated system uses a commercially available inertial filter to separate the particles from the gas stream, but it plugs off after less than thirty days.
U.S. Pat. No. 4,259,867 describes an apparatus for sampling pyrolysis gas from a hydrocarbon cracking furnace, but needs condensable materials in the gas stream to wash particles from a packing material. Even the smallest amount of water, condensate, or other polar volatile liquids in a pipe attached to a hot, gas-filled process line may cause cracking of the process piping through stress corrosion as the liquid is vaporized at the metal surface. This risk of stress corrosion prevents us from adapting a commercial version of the pyrolysis-gas sampling apparatus to FCC flue-gas service.